1. Field of the Invention
The present invention relates to a method and apparatus for measuring the intensity decay time of an optical cavity at various wavelengths. More particularly, this invention pertains to such a method and apparatus of particular utility with regard to cavities of the type that include low loss mirrors.
2. Description of the Prior Art
Numerous optical instruments are critically dependent upon mirror parameters. For example, in a ring laser gyroscope, mirrors located at the intersections of three or four cavities internal to a glass ceramic block successively reflect counter-rotating beams of laser light that, upon transmission through a partially-transmissive mirror, are analyzed for frequency content. It is essential that the operating parameters of the mirrors of such an instrument be well-known to permit accurate evaluation of the optical output of this device. Numerous other applications require detailed knowledge of the system mirrors. Further, one may design improved optical systems when detailed a priori knowledge of the mirror characteristics, and the thin films for forming mirror surfaces, is available.
The mirrors generally employed in high accuracy laser instrumentation commonly comprise a substrate having an optical coating thereon. This coating may be applied in a plurality of layers. For example, in the fabrication of mirrors for current state-of-the art ring laser gyroscopes, a number of thin film coating layers, each having a thickness of about 1000 Angstroms or one-quarter wavelength, are applied to a glass ceramic substrate. The number of layers applied may vary from 10 to 50, with most coatings comprising about 21 or 22 layers.
While the mirrors can play a crucial role in the performance of such optical instruments, coating characteristics may vary considerably from manufacturer to manufacturer, even in the presence of identical specifications. Thus, it is highly desirable to have and utilize a device for measuring the actual characteristics of the mirror and the constituent thin films.
A theoretical framework for measuring the loss of highly reflecting mirror coatings and transmission of low-loss antireflection coatings is disclosed by Dana Z. Anderson, Josef C. Frisch and Carl S. Masser in "Mirror Reflectometer Based on Optical Decay Time", Applied Optics., Vol. 23, No. 8 (Apr. 15, 1984). The artical establishes that total mirror loss (the sum of scattering, transmission and absorption losses) may be found by analysis of its operation within an optical cavity. (An optical cavity may be defined as an enclosed cavity wherein light is directed between at least two mirrors.) The total mirror loss may then be established from the cavity intensity decay time c, the optical path length, L, of the cavity and the speed of light c. Once the decay time cavity number is known, other important mirror parameters, including finesse, reflectivity and reflectivity product may be easily derived.
The measurement of cavity lengths is relatively trivial and the speed of light a known value. The above-mentioned article proposes a method for determining the cavity time constant by brief laser energization of the cavity followed by timing of the decay in the intensity of the light within the cavity (measured as transmitted through a partially-transmissive cavity mirror). A very narrow bandwidth laser is used to generate the energy for exciting cavity resonance. This laser is always "on" and a short burst of light therefrom is shuttered through crossed polarizers by means of a Pockels cell. The narrow-band laser is allowed to drift in frequency, occasionally and randomly drifting to a resonant frequency of the cavity. (The article suggests that sweeping or frequency control of the narrow laser are thereby avoided.) The cavity is energized so that a measurable amount of light will be detected by the system electronics as the laser drifts to a cavity resonant frequency. When this occurs, a photodetector detects the light intensity within the cavity, the laser is shuttered, and the decay time of the cavity measured.
While the method is theoretically sound, the apparatus as disclosed above is flawed in a number of respects of particular significance with regard to the accuracy required in the measurement of low-loss mirrors. A high reflectively or low-loss mirror is one which loses or transmits less than 200 parts per million of incident illumination. The optical shutter of the "on" laser admits an irreducible and difficult-to-ascertain amount of "baseline" illumination at all times, "on" or "off". This illumination prevents the user from knowing the precise intensity values for the start and stop points between which the cavity decay time is counted. Since only very low levels of illumination are transmitted by low-loss mirrors, such background effects are particularly harmful when measuring highly reflective mirrors. Additionally, by allowing the laser to drift into and out of the resonant frequency of the cavity, false readings can occur that result from the sometime nonmonotonic nature of the intensity decay curve for a cavity energized by a laser that may rapidly drift into and out of cavity resonance, causing re-excitation of the cavity to occur after the initiation of the "exponential" decay timing has been signalled to the electronic timing subsystem of the test apparatus.